The present invention relates to power back-up units, and more particularly to a power back-up unit with low voltage disconnects that disconnect or shed one load while maintaining power to a second load.
Telecommunications, cable television, power distribution equipment and the like are all attached to various electrical loads such as telephones, televisions, appliances, etc. During normal operation these loads are supplied with ac power. These loads can also be connected to a secondary power source such as a battery or generator. This secondary power source is employed to provide reserve energy to the electrical loads in the event of a power outage or fault. Power back-up systems are one way to provide reserve energy in the event of a loss of primary power.
Certain of these back-up systems include a connection to an ac power source, a battery, a low voltage disconnect (LVD) and a connection to one or more loads. The battery is generally maintained in a state of readiness, i.e., fully charged. The battery is generally connected to a rectifier that converts the incoming ac power to DC power. The load(s) are connected to the battery and the rectifier; however, the rectifier normally supplies power to the electrical load(s). If, however, the DC power from the rectifier is interrupted (e.g., by a loss of ac power or a rectifier malfunction), then the secondary power source (e.g., battery) will supply power to the load. The uninterrupted supply of power is an important consideration in the design of lifeline support systems. Even though commercial ac power is typically available about 99.9% of the time, reserve power is required for uninterruptable systems. Power can be disrupted, for example, due to severe weather or an equipment failure. This interruption causes the electrical load(s) to either shut down or switch to a secondary power source. In systems where the electrical loads switch to battery power, one or more of the loads can drain the reserve battery before the ac power is restored. However, it is highly undesirable to allow a battery to discharge completely because, if this happens, it becomes impractical to recharge the battery. Thus, the battery is usually discarded. It is far better to only partially discharge a battery so as not to completely drain the battery and permanently damage it. Therefore, certain prior battery back-up units provide a low voltage disconnect (LVD) that monitors the output voltage of the battery and, when that output voltage drops below a set threshold, disconnects the load(s) from the battery to prevent draining the battery to the point where the battery is permanently damaged.
However, these prior battery back-up units are deficient in a number of ways. For example, when one or more loads drain a battery beyond the set threshold of a prior battery back-up unit before the ac power is restored, the LVD disconnects all of the loads from the battery thereby shutting down all of the loads. This is unacceptable where one of the loads is a critical load such as a lifeline support system. A lifeline support system can include plain-old-telephone-service (POTS). Typically, every home and business has a primary telephone line that provides 911 emergency service. Accordingly, there is a need for a load voltage disconnect (LVD) that allows a critical load (such as a lifeline support system) to maintain its connection with a secondary power source while disconnecting one or more non-critical loads (such as an asymmetrical digital subscriber line (ADSL) system) from the secondary power source so as to prolong the time period that the secondary power source can operate the critical load.
A power back-up unit is provided which includes a primary power source that is connected to a plurality of loads, including a critical load and a non-critical load. A secondary power source is also provided. A first low voltage disconnect is electrically connected to the secondary power source and the critical load. A second low voltage disconnect is electrically connected to the secondary power source and the non-critical load. In one embodiment, the second low voltage disconnect disconnects the non-critical load upon loss of the primary power source. In another embodiment, the second low voltage disconnect has a voltage threshold that is greater than the voltage threshold of the first low voltage disconnect such that the non-critical load is disconnected by the second low voltage disconnect prior to the critical load being disconnected by the first low voltage disconnect.
Additional novel features and advantages of the present invention will be set forth in part a in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed.